Pneumatic cylinder and control valve therefor

Abstract: A pneumatic cylinder, especially for a spot-welding machine, has a sleeve valve mounted on the piston rod. The sleeve valve provides a venting valve 54,56 for dumping pressure from return chamber 34 at the end of the forward stroke, and a progressive restrictor 46,58 to close off forward chamber 32 and provide cushioning at the end of the return stroke.

Description: This invention relates to a method of operating a pneumatic cylinder, pneumatic cylinders and control valves for such cylinders. It is particularly applicable to cylinders for use in for example spot welding machines, piercing machines, powered jigs or clamps. There already exist various types of pneumatic cylinders with either internal or external flow controls and cushioning devices. For some types of heavy duty production operations these typical designs do not meet specific production needs. This is especially true of pneumatic cylinders used in resistance spot welding and some piercing or clamping operations. For example, in resistance spot welding an air cylinder with an electrode and associated equipment attached to the piston rod is typically expected to stroke forward, contact the component and reach full welding force in 0.2 secs. It must precisely hold this force for the welding and cooling period of typically 0.4 secs and during this period rapidly follow up at constant force the collapse or indentation of the components during welding. To achieve this fast and then stable force condition it is usual to ensure that air can be supplied to and exhausted from the power cylinder very quickly. In addition the end of the forward stroke is determined not by the cylinder but by the contact point of the electrode which changes during its production life as the electrode is consumed, worn, re-profiled or replaced. This combination of rapid force build up and variable end of stroke positioning make traditional cushioning and speed control devices difficult to apply, and without them the shock loads, noise, wear and vibration of tooling and equipment in use is considerable. One such cylinder is shown in US-A-5.174.191. In that cylinder, the above-described functions are achieved at the penalty of using a large number of components, leading to a complex and expensive device. DE-A 3542069 discloses the preamble of claim 1, 2 and 14. The preferred embodiments of the present invention have as one object the provision of an alternative solution using a much simpler and less expensive construction. Another object of those embodiments is to provide a cylinder in which the movement of the piston is braked at the end of its return stroke. In this specification the term "piston" is used generally to include the piston heads and piston rods together making up a piston assembly moveable in a cylinder body. Similarly, "pneumatic" whilst referring primarily to compressed air also includes other compressed gases used as a power source. In a first aspect, the invention provides a method of operating a pneumatic cylinder comprising a cylinder body containing a piston and defining therewith first and second pressure chambers such that pressure fluid supplied to the first chamber effects relative movement of the piston and cylinder in a first sense, and pressure fluid supplied to the second chamber effects relative movement of the piston and cylinder in the opposite sense, the method comprising restrictedly venting the second chamber during relative movement in the first sense, and upon the said relative movement in the first sense being arrested then venting the pressure fluid in the second chamber by operating a moveable valve element, and characterised by progressively restricting a flow of said pressure fluid venting from the first chamber or restricting a flow of said pressure fluid being supplied to the second chamber by means of the same valve element when relative movement of the piston and cylinder in the opposite sense approaches a limiting extent thereof whereby to moderate said relative movement. In another aspect the invention provides a pneumatic cylinder comprising a cylinder body containing a piston and defining therewith first and second pressure chambers such that pressure fluid supplied to the first chamber effects relative movement of the piston and cylinder in a first sense, and pressure fluid supplied to the second chamber effects relative movement of the piston and cylinder in the opposite sense, means providing restricted venting of the second chamber during relative movement in the first sense, and valve means operative upon said relative movement in the first sense being arrested for then venting the pressure fluid in the second chamber, characterised in that the valve means also is operative to progressively restrict a flow of the pressure fluid venting from the first chamber or to restrict a flow of pressure fluid being supplied to the second chamber when relative movement of the piston and cylinder in the opposite sense approaches a limiting extent thereof whereby to moderate said relative movement. The means for providing restricted venting may be adjustable to control the speed of relative movement in the first sense. The valve means may be operative to restrict the flow of pressure fluid from the first chamber and to restrict the flow of pressure fluid to the second chamber when the relative movement in the opposite sense approaches said limiting extent. The valve means may comprise a first portion cooperating with fixed structure of the cylinder body to define a venting valve which opens to vent the second chamber, and a second portion cooperating with the piston to define a restrictor valve which progressively closes during relative movement in the opposite sense. The valve means may be a sleeve valve carried by the piston coaxially therewith. In one preferred embodiment, the first portion of the valve means comprises a flange on the sleeve valve having a radially extending sealing surface, the fixed structure of the cylinder body having a shoulder with a matching sealing surface. The flange and shoulder may also serve to restrict the supply of pressure fluid to the second chamber, as set forth above. The restrictor valve may comprise a portion of the sleeve valve having an axially extending tapered bore through which the piston passes, the piston having a port on a circumferential surface which communicates with the first chamber via the tapered bore, progressive restriction of venting of the first chamber being effected by relative movement of the port in the tapered bore towards the smaller end thereof. The cylinder may have a third pressure chamber adapted to be pressurised throughout during relative movement of the piston and cylinder in the first sense, to effect said movement. It may communicate with the first chamber via a conduit in the piston which is connected to said port. A further disadvantage of known power cylinders is that the control mechanism is located within the cylinder, again increasing complexity and cost. In another aspect of the invention, this disadvantage is avoided. According to this aspect of the invention, there is provided a control valve for a pneumatic cylinder as described in claim 14. This aspect of the invention is particularly suitable for simple pneumatic power cylinders with a single piston, and also to improve equipment not fitted with a low impact cylinder. The control valve may be a separate unit or it may be integrated as convenient with a cylinder head or body, as appropriate for a specific application. Thus, another preferred embodiment of the cylinder is described in claim 12. The restricted bleed conduit may be formed by a passage in the valve spool, or by a passage through the valve body. The bleed conduit may be adapted to provide both restricted venting of the second chamber during a forward stroke of the piston of the pneumatic cylinder, and to restrict the supply of pressure fluid to the second chamber as the piston approaches the end of its return stroke. The invention also includes a control valve as set forth above in combination with or integrated with a pneumatic cylinder having a piston and first and second oppositely-acting pressure chambers. The invention will be described merely by way of example with reference to the accompanying drawings, wherein: Figures 1 to 4 show successive stages in the operation of a cylinder according to the invention; Figure 5 shows a control valve according to the invention; Figures 6 and 7 show modified forms of the pneumatic cylinder; and Figures 8 and 9 show modified forms of the control valve. Referring to Figure 1, a pneumatic cylinder comprises a cylinder body 10 having therein a piston assembly 12. The piston assembly has two piston heads 14,16 and piston rods 18,20. The piston rod 18 is supported in the end wall 22 of the cylinder body 10 and in this (prototype) example is provided with an internally threaded bore 24 enabling the piston rod to be connected to for example, a spot-welding head. Normally, the end of the piston would conform to the ISO standard for tooling and electrode mountings. The wall 22 has a vent hole 26 to prevent a build-up of pressure in the otherwise-closed space 28 behind the piston head 14. The cylinder body 10 has an intermediate wall 30 which defines first and second pressure chambers 32,34 with opposing faces 36,38 of piston heads 14,16. A third chamber 40 is defined between the other face of piston head 16 and the end 41 of the cylinder body 10. An internally-threaded port 42 for connection to a source of compressed air or other pressurised gas is provided to chamber 40 to drive the piston in its working stroke. Chamber 40 communicates via an axial bore 44 and ports 46 in the circumferential surface of piston rod 20 with chamber 32. Chamber 34 is provided with a port 48 having an internal thread 47 for the supply of compressed air to piston head 16 to drive the piston assembly in its return stroke. A bleed hole 50 provides restricted venting of chamber 34 during the forward (power) stroke of the piston assembly 12. A sleeve 52 is a close sliding fit on piston rod 20 within the intermediate wall 30, and has at one end a flanged portion 54, one radially extending face of which forms a venting valve with a shoulder 56 on the intermediate wall 30. An annular space 57 permits flow between chamber 34 and return port 48 when the venting valve 54,56 is open. At the other end of the sleeve 52, the internal bore of the closure has an enlarged tapering portion 58 which cooperates with the ports 46 to form a progressively variable restriction in the conduit between chambers 32 and 40, as hereafter described. To permit assembly of the piston assembly 12 in the cylinder the piston heads 14 and 16 are separable from the piston rod 20, and the end 22, and intermediate wall 30 of the cylinder body each are removable. A circlip 60 limits the travel of the sleeve 52 within the intermediate wall. Grooves 62,64,66,68 contain piston rings, O-rings or other suitable conventional seals (not shown) to minimise unwanted leakage from or between the chambers of the cylinder body 10. The operation of the cylinder will now be described. In Figure 1, the piston is fully retracted and a conventional air directional valve (not shown) maintains this position by exhausting chambers 32 and 40 to atmosphere and supplying air under pressure to chamber 34. The venting valve 54,56 is maintained closed by the pressure in chamber 34. In Figure 2, the air directional valve is energised, chambers 32 and 40 are pressurised and the return port 48 is exhausted to atmosphere. The piston 12 moves leftwards relatively to the cylinder 10 towards a workpiece 69, the speed of movement being controlled by the size of the bleed hole 50 and any leakage past venting valve 54,56. Relatively high pressure is maintained in chamber 34 by the piston movement under the working pressure in chambers 32,40, against the resistance of bleed hole 50 thereby maintaining venting valve 54,56 closed. In Figure 3, the piston 12 has ceased moving due to the workpiece having contacted an electrode, punch, clamp or other tool. The pressure in chamber 34 continues to fall as air discharges through bleed hole 50, and the pressure in chambers 32,40 quickly increases up to the maximum available supply pressure. Under the effect of the change in pressure difference between chambers 32 and 34 the sleeve 52 moves to the right, opening venting valve 54,56 and rapidly exhausting the air in chamber 34 to atmosphere through the directional air valve. To effect the return stroke the air directional valve is de-energised, exhausting the air in chambers 32 and 40 whilst supplying pressurised air to chamber 34. Initially the higher pressure in chamber 32 relative to chamber 34 holds the venting valve 54,56 open, allowing compressed air rapidly to enter chamber 34. As chamber 32 exhausts the balance of pressure changes and the sleeve 52 moves left, closing venting valve 54,56, and restricting the flow of air into chamber 34 to that passing through bleed hole 50. Referring to Figure 4, as the piston assembly 12 retracts into the cylinder body 10, the ports 46 become progressively restricted as they enter the tapered bore 58 of the sleeve 52. The restricted supply of pressurised air to chamber 34 together with the progressive increase in pressure in chamber 32 due to the increasing restriction of the ports 46 retards the rightward movement of the piston 12 as it approaches the end of its stroke, and cushions or eliminates the impact between the piston and cylinder body at the end of the stroke. Figure 5 shows a control valve for use with a conventional double-acting single or dual piston pneumatic cylinder without internal valves. The control valve comprises a valve block 70 in which is disposed a valve spool 72. Forward and return ports are intended to be connected to the forward and return chambers of the cylinder, and are thus numbered 42 and 48, as in Figure 1. The port 42 communicates with a first open conduit 74 through the valve block, and the port 48 communicates with a second conduit 76 through the block which is controlled by the valve spool. The valve spool has a flange 54 which similarly to Figure 1 forms a venting valve with a shoulder 56 of the valve block; these ports are numbered as in Figure 1 to indicate similarity of function. A restricted bleed conduit 50 passes through the valve spool, by-passing the venting valve 54,56 so as to be functionally equivalent to the bleed hole 50 of Figure 1. Oppositely facing pressure surfaces 80,82 on the spool 72 are exposed to the pressure at the ports 42,48 respectively. When fitted to a double-acting cylinder the valve of Figure 5 controls the cylinder to operate on its forward stroke similarly to the cylinder of Figure 1. Thus the bleed conduit 50 maintains a back-pressure in the return chamber of the cylinder holding venting valve 54,56 closed until the piston rod contacts the workpiece and stops. Then the pressure at port 48 decays, enabling the pressure in conduit 74 to move the spool to the right, opening the venting valve and dumping the remaining pressure in conduit 48 and the cylinder chamber connected thereto. On the return stroke, the forward air pressure at port 42 is removed and the pressure in the forward chamber rapidly decays. The venting valve 54,56 is at first open allowing a fast flow of pressurised air through port 48 to the return chamber of the cylinder. As the cylinder forward air pressure reduces and the return air pressure increases the venting valve closes, and the supply of air to the return side of the cylinder becomes restricted to that passing through the bleed conduit 50. The practical effect of this is to return very quickly the piston assembly to its initial position, but under reducing pressure which gives a noticeable reduction of impact force when the mechanism reaches its limit-stop at the end of the return stroke. The invention may be applied in both the forms shown in Figures 1 to 4 and Figure 5 to multiple pistons. Such multiple piston arrangements may be achieved as shown in Figures 1 to 4, in which forward pressure is applied to the multiple pistons via bores in the piston rods, and return pressure either by a single return port as shown or by separate return ports. Thus in Figure 6 (in which already-described parts carry the same reference numerals) a modified form of the apparatus has in addition piston 16' on an extended piston rod 20' and bounding chambers 34' and 40'. Forward port 42 communicates with chamber 40' and chamber 40 is in this embodiment provided with pressure-fluid through extended bore 44' and port 46' in the piston rod 20'. An additional return port 48' is provided for chamber 34', but if preferred only the one port 48 need be provided, chamber 34' then instead being provided with a vent similar to vent 26 of chamber 28. Increasing the number of pistons enables the force applied during the forward (working) stroke for a given working pressure to be increased without increasing the cylinder bore. Figure 6 also shows a modification to the restricted bleed hole 50. The internal thread 47 of port 48 is extended as far as the intersection of bleed hole 50 with the port, and a hollow sleeve 49 is fitted therein. The sleeve 49 has a hexagonal section recess 51 enabling the sleeve to be screwed in or out of the threaded bore 48 by means of an Allen key, so that its lower end occludes the bore 50 to a greater or lesser extent. The degree of restriction provided by the bleed hole 50 is thereby adjustable, with the result that the speed of the piston assembly 12 during its forward stroke may be controlled. In the double acting cylinder of Figure 7 (which is otherwise as shown in Figure 1) the bleed hole 50 is shown bored in the lower portion of the intermediate wall 30, 180° round from the return port 48. The bleed hole 50 communicates with a radially-extending tapped bleed conduit 90 which in turn communicates with the annular space 57 and thus with the return port 48. An externally adjustable bleed screw 92 is threaded into the bleed conduit 90 partially to occlude a portion of the bleed hole 50, thereby providing an externally accessible means to control the degree of restriction of the bleed hole 50. The end of the screw 92 is in the form of a truncated cone 94 to permit a finer adjustment of the restriction of the bleed hole 50. The degree of taper is exaggerated in the drawing, for clarity. A tapered pin 96 is provided across one side of the outer end of the conduit 90 to prevent the bleed screw being withdrawn too far, whilst still permitting access to the head of the screw for adjustment thereof. If withdrawn too far, insufficient threads of the screw would remain engaged with the bore 90, and the screw may be ejected forcibly and at high velocity by the air pressure in the cylinder. A lip-type seal 98 around the shank of the screw 92 beneath the head minimises air leakage from the conduit 90 and prevents the ingress of foreign matter. It will be appreciated that the bleed hole 50 and conduit 90 need not be 180° round from the return port 98, but may be at any convenient angle consistent with simplicity of manufacture. Figures 8 and 9 show similar modifications applied to the control valve of Figure 5. In Figure 8, at least an end portion of the bleed conduit 50 of the valve spool is internally threaded at 47, and a hollow sleeve 49 with a hexagonal section bore is fitted therein. The sleeve 49 can be screwed in or out partially to occlude a radial portion 86 of the bleed conduit 50, so as to adjust the degree of restriction afforded by the conduit. Access to the sleeve 49 for adjustment is obtained by removing the end plate 88 of the valve body. The control valve of Figure 9 provides an externally accessible means of controlling the pressure in the cylinder return port 48. The bleed conduit 50 of Figure 8 is replaced by bores 106,108 in the valve block 70 which intersect to form an orifice 110. Bore 106 communicates with conduit 86 and bore 108 communicates with return port 48. The bore 106 is tapped and contains a bleed adjusting screw 104 having a tapered end, which depending on the position of the screw, occludes the orifice 110 to a greater or lesser extent. The bore 106 passes through the end plate 88, so that the other (outer) end of the screw 104 is accessible, enabling its position and the degree of bleed through bores 106,108 to be adjusted without dismantling the valve. The outer end of the bore 108 is closed by a plug 112. For applications such as resistance spot welding, the fixed bleed hole embodiments of Figures 1 to 5 are preferred so that every cylinder can have a predetermined production performance. For other applications such as clamping, the variable bleed embodiments will often be preferred. Both the integral sleeve valves of Figures 1 to 4, 6 and 7 and the separate control valves of Figures 5, 8 and 9 may be set to operate at chosen and predetermined forward and return pressure balances across the sleeve or valve spool. This is achieved by appropriate choice at the design stage of the diameters of the relevant pressure faces. For most applications however the pressure balance is not critical.

Claims: A method of operating a pneumatic cylinder comprising a cylinder body (10) containing a piston (12) and defining therewith first (32) and second (34) pressure chambers such that pressure fluid supplied to the first chamber effects relative movement of the piston and cylinder in a first sense, and pressure fluid supplied to the second chamber effects relative movement of the piston and cylinder in the opposite sense, the method comprising restrictedly venting the second chamber during relative movement in the first sense, and upon the said relative movement in the first sense being arrested then venting the pressure fluid in the second chamber by operating a moveable valve element, and characterised by progressively restricting a flow of said pressure fluid venting from the first chamber or restricting a flow of said pressure fluid being supplied to the second chamber by means of the same valve element when relative movement of the piston and cylinder in the opposite sense approaches a limiting extent thereof whereby to moderate said relative movement. A pneumatic cylinder comprising a cylinder body (10) containing a piston (12) and defining therewith first (32) and second (34) pressure chambers such that pressure fluid supplied to the first chamber effects relative movement of the piston and cylinder in a first sense, and pressure fluid supplied to the second chamber effects relative movement of the piston and cylinder in the opposite sense, means (50) providing restricted venting of the second chamber during relative movement in the first sense, and valve means (52, 54, 72) operative upon said relative movement in the first sense being arrested for then venting the pressure fluid in the second chamber, characterised in that the valve means also is operative to progressively restrict a flow of the pressure fluid venting from the first chamber or to restrict a flow of pressure fluid being supplied to the second chamber when relative movement of the piston and cylinder in the opposite sense approaches a limiting extent thereof whereby to moderate said relative movement. A cylinder as claimed in Claim 2, wherein the means (49, 50) for providing restricted venting is adjustable to control the speed of relative movement in the first sense. A cylinder as claimed in Claim 2 or Claim 3, wherein the valve means (52, 54) is operative both to restrict the flow of pressure fluid from the first chamber and to restrict the flow of pressure fluid to the second chamber (34) when the relative movement in the opposite sense approaches said limiting extent. A cylinder as claimed in Claim 4, wherein the valve means comprises a first portion (54) cooperating with fixed structure (56) of the cylinder body to define a venting valve which opens to vent the second chamber, and a second portion (58) cooperating with the piston to define a restrictor valve which progressively closes during relative movement in the opposite sense. A cylinder as claimed in any of Claims 2 to 5, wherein the valve means is a sleeve valve (52) carried by the piston coaxially therewith. A cylinder as claimed in Claims 5 and 6, wherein the first portion of the valve means comprises a flange (54) on the sleeve valve (52) having a radially extending sealing surface, the fixed structure of the cylinder body having a shoulder (56) with a matching sealing surface. A cylinder as claimed in Claim 7, wherein the flange and the shoulder (54, 56) also restrict the supply of pressure fluid to the second chamber. A cylinder as claimed in Claims 5 and 6, wherein the restrictor valve comprises a portion of the sleeve valve having an axially extending tapered bore (58) through which the piston passes, the piston having a port (46) on a circumferential surface which communicates with the first chamber (32) via the tapered bore, progressive restriction of venting of pressure fluid from the first chamber being effected by relative movement of the port in the tapered bore towards the smaller end thereof. A cylinder as claimed in any of Claims 2 to 9 comprising a third pressure chamber (40) adapted to be pressurised throughout said relative movement in the first sense to effect said relative movement. A cylinder as claimed in Claims 9 and 10 wherein the third pressure chamber communication with the first pressure chamber via a conduit (44) in the piston which is connected to said port (46). A cylinder as claimed in Claim 2, 3 or 4 wherein the valve means comprises a valve spool (72) moveable axially of itself in a valve block (70) the valve block and spool defining a first conduit (74) through the valve block and adapted to conduct pressure fluid to the first pressure chamber, a second conduit (76) through the valve block controlled by valve surfaces of the valve block and spool and adapted to conduct pressure fluid to the second pressure chamber, oppositely facing pressure surfaces (80, 82) of the spool being respectively exposed to pressure fluid in the respective conduits such that pressure in the second chamber biases the spool valve to maintain the second conduit closed, the restricted venting means being a bleed conduit (50) by-passing the valve surfaces in the second conduit, pressure fluid being supplied to the second chamber when motion in the second sense approaches its limiting extent only through the bleed conduit (50). A cylinder as claimed in Claim 12 wherein the valve block is integrated with a cylinder head or body of the cylinder. A control valve for a pneumatic cylinder having a piston, a first chamber to be pressurised to effect a forward stroke of the piston and second oppositely-acting chamber to be pressurised to effect a return stroke of the piston, the control valve comprising a valve block (70) with a valve spool (72) moveable axially therein, and characterised by the valve block and spool defining a first conduit (74) through the valve block and adapted to conduct pressure fluid to the first pressure chamber of the cylinder, a second conduit (76) through the valve block controlled by valve surfaces of the valve block and spool and adapted to be connected to the second pressure chamber of the cylinder, oppositely facing pressure surfaces (80, 82) of the spool being respectively exposed to pressure fluid in the respective conduits such that pressure applied to the downstream end of the second conduit biases the spool valve to maintain the second conduit closed, and a restricted bleed conduit (50) by passing the valve surfaces in the second conduit and adapted to provide both restricted venting of the second chamber during the forward stroke of the piston, and to permit a restricted supply of pressure fluid to the second chamber as the piston approaches the end of its return stroke following closure of the second conduit. A cylinder as claimed in Claim 12 or 13, or a control valve as claimed in Claim 14, wherein the degree of restriction provided by the restricted bleed conduit is adjustable. A cylinder as claimed in Claim 12, or a control valve as claimed in Claim 14 or 15, wherein the restricted bleed conduit is formed by a passage (50) through the valve spool (72). A cylinder as claimed in Claim 12, or a control valve as claimed in Claim 14 or 15, wherein the restricted bleed conduit is formed by a passage (106, 108) through the valve block. A pneumatic cylinder having a first pressure chamber, a second oppositely-acting pressure chamber, and in combination with or integrated with a control valve as claimed in any of Claims 14 to 17.

Inventors: TRIMMER DAVID JOHN, WARD ALAN, TRIMMER, DAVID JOHN, WARD, ALAN, TRIMMER, DAVID JOHN, C/O SAVAIR LIMITED, WARD, ALAN, C/O SAVAIR LIMITED
Applicants: SAVAIR & ARO LTD, SAVAIR + ARO LIMITED
Application year: 1994
Main classification: F15B11
IPC codes: F15B15, F15B11, B23K11